Photocurable ionomer cement systems

ABSTRACT

A dental cement system containing a photocurable ionomer, reactive powder and water undergoes both a conventional setting reaction and a photocuring reaction. 
     The cement system can provide a long working time and can be cured on demand by exposure to an appropriate source of radiant energy. The surface of the cement cured in this manner is then hard enough to allow subsequent clinical procedures to be performed, while the ongoing chemical-cure &#34;setting&#34; reaction hardens the remainder of the cement.

This is a continuation of application Ser. No. 08/729,830 filed Oct. 8,1996, now abandoned, which is a continuation of Ser. No. 07/605,749filed on Oct. 30, 1990, now abandoned, which is a division of Ser. No.07/139,387 filed Dec. 30, 1987, which is abandoned.

TECHNICAL FIELD

The present invention relates to ionomer cement systems useful, forinstance, for the preparation of dental and medical adhesives, bases,liners, luting agents, sealants, and filling materials for restorativeand/or endodontic use. This invention also relates to methods andcompositions for setting and curing such cement systems. In anotheraspect this invention relates to the ionomers useful in such systems, aswell as to methods of using such ionomers, and the cements formed withsuch ionomers.

BACKGROUND ART

The setting reaction of ionomer cements is known mainly through studiesof glass ionomer cements, i.e., ionomer cements in which the powder usedin the cement is an ion-leachable glass, such as those based on calciumaluminosilicate glasses, or more recently, borate glasses. Seegenerally, Prosser et al., "Polyelectrolyte Cements", Wilson andProsser, eds., Developments in Ionic Polymers--1, Chapter 5, AppliedScience Publishers (London and New York, 1983). In the setting reaction,the powder behaves like a base and reacts with the acidicpolyelectrolyte, i.e., ionomer, to form a metal.polysalt which acts asthe binding matrix. Water serves as a reaction medium and allows thetransport of ions in what is essentially an ionic reaction.

The setting reaction is therefore characterized as a chemical curesystem that proceeds automatically upon mixing the ionomer and powder inthe presence of water. The cements set to a gel-like state within a fewminutes and rapidly harden to develop strength. See, e.g., Prosser etal., J. Chem. Tech. Biotechnol., 29, 69-87 (1979). Chelating agents,such as tartaric acid, have been described as useful for modifying therate of setting, e.g., to provide longer working times for the cements.See, e.g., U.S. Pat. Nos. 4,089,830, 4,209,434, 4,317,681 and 4,374,936.Unfortunately, when working times are lengthened by the usual methods,setting times are generally also lengthened.

Many commercially available glass ionomer cements include such chelatingagents, and as a result are characterized by working times that are onthe order of 1 to 2 minutes, but relatively long setting times, e.g., onthe order of 4 to 15 minutes. During this set time a dry field must bemaintained, and yet dessication of the cement must be avoided. Suchconditions can lead to discomfort for the patient as well as the addedburden of having to spend extra time in the dentist's chair. Thuspresent day glass lonomer cements, although beneficial clinically, arequite technique-sensitive, as well as time-consuming for the dentist andpatient.

Of peripheral relevance to the present invention, but worth notingnonetheless, Mathis et al., J. Dent. Res., 66:113 (Abst. No. 51) (1987),reports the addition of a separate light curable composite resin to theliquid component of an ionomer in order to form a "hybrid" material.This hybrid material was cured by exposure to a visible light sourceimmediately after mixing it with powder.

SUMMARY OF THE INVENTION

Further adjustability of working time and setting time would bedesirable in order to provide greater flexibility in the formulation ofionomer cement systems, and in particular, glass ionomer cement systems.Such adjustability is also desirable in order to extend the practicalapplication of such cement systems to uses involving higher glassloading levels (e.g., for posterior or incisal applications) or lowermix viscosity (e.g., endodontic sealants) than are attainable usingcurrent techniques.

The present invention provides, in one aspect, ionomer cement systemsthat are photocurable using radiant energy. Such systems provide theopportunity to achieve long working times as well as short settingtimes. These systems are prepared using photocurable ionomers whichcomprise a polymer having sufficient pendent ionic groups to undergo asetting reaction in the presence of a reactive powder and water, andsufficient pendent polymerizable groups to enable the resulting mixtureto be cured by exposure to radiant energy.

The invention also provides methods for preparing and methods for usingsuch photocurable ionomer cement systems. In another aspect the presentinvention also provides novel photocurable ionomers.

The photocurable ionomer cement system of the present inventioncomprises (a) photocurable ionomer as described herein, and (b) reactivepowder. Preferred optional ingredients of the photocurable ionomersystem include water (present in a form that does not prematurely beginto set the system), appropriate polymerization initiators, modifyingagents, and copolymerizable and non-copolymerizable cosolvents. Otheroptional ingredients include pigments, fillers (e.g., pulverizedprecious or nonprecious metals, silica, quartz or metal oxides), and thelike.

The photocurable ionomer cement systems of the present invention can beused to prepare a cement by combining the ionomer and the reactivepowder in the presence of water. As with present day cement systems, thewater serves as a reaction medium allowing the transport of ions betweenthe ionomer and the reactive powder, thereby allowing the acid-basechemical cure "setting" reaction to occur. This setting reaction canalso be termed the "dark reaction" in that it will proceed regardless ofthe presence of light or any other form of radiant energy.

The systems of the present invention provide a valuable and time-savingopportunity to cure the system rapidly and on demand by a brief exposureto an appropriate source of radiant energy after the necessarycomponents have been mixed and the setting reaction has begun, but whilethe cement is still in a fluid or plastic, i.e., "workable", state. As aresult, the practitioner can achieve long working times by the use ofmodifying agents to slow the setting reaction, but need not be burdenedwith the typical correspondingly long setting times. By the use ofmodifying agents as discussed in greater detail below, the resultantsystems can be made to have sufficiently long working times for use innew medical and dental applications where ionomer cements have nothitherto been employed.

Applicant has also discovered that certain modifying agentstraditionally used in glass ionomer cement systems, e.g., tartaric acid,do not provide longer working times when used in the systems of thepresent invention. Applicant has discovered that a new group ofcompounds can be used as modifying agents in order to provide thedesired result of prolonged working times.

Moreover, by the use of fluoride-containing reactive powders, asexplained more fully below, the present invention provides the abilityto prepare a dental restorative that is both photocurable and capable ofexhibiting cariostatic fluoride release. Such a combination ofproperties is highly desirable.

DETAILED DESCRIPTION

The term "photocurable ionomer", as used herein, refers to a polymerhaving sufficent pendent ionic groups to undergo a setting reaction inthe presence of a reactive powder and water, and sufficient pendentpolymerizable groups to enable the resulting mixture to be polymerized,i.e., cured, upon exposure to radiant energy.

The term "reactive powder", as used herein, refers to a metal oxide orhydroxide, mineral silicate, or ion-leachable glass that is capable ofreacting with the ionomer in the presence of water to form a hydrogel.

The term "ionomer cement system", as used herein, refers to the unmixed,or mixed but unset and uncured, combination of photocurable ionomer,reactive powder, and other optional ingredients, such as water. Suchsystems include kits in which the ionomer is employed as a concentratedaqueous solution, for mixing directly with the powder, as well as kitsin which the ionomer is employed in a dry blend with the powder, forlater mixing with water.

The term "working time", as used herein, refers to the time between thebeginning of the setting reaction, i.e., when the ionomer and reactivepowder are combined in the presence of water, and the time the settingreaction has proceeded to the point at which it is no longer practicalto perform further physical work upon the system, e.g., spatulate it orreform it, for its intended dental or medical purpose.

The term "setting time", as used herein, refers to the time between thebeginning of the setting reaction in a restoration, and the timesufficient hardening has occured to allow subsequent clinical proceduresto be performed on the surface of the restoration. Such hardening canoccur either in the course of the normal setting reaction and/or bycuring a photocurable system.

Photocurable ionomers of the present invention comprise a polymer havingsufficient pendent ionic groups to undergo a setting reaction in thepresence of a reactive powder and water, and sufficient pendentpolymerizable groups to enable the resulting mixture to be cured byexposure to radiant energy.

Preferred photocurable ionomers have the general Formula I:

    B (X) m(Y) n I

wherein

B represents an organic backbone,

each X independently is an ionic group capable of undergoing a settingreaction in the presence of water and a reactive powder,

each Y independently is a photocurable group,

m is a number having an average value of 2 or more, and

n is a number having an average value of 1 or more.

Preferably the backbone B is an oligomeric or polymeric backbone ofcarbon-carbon bonds, optionally containing non-interfering substituentssuch as oxygen, nitrogen or sulfur heteroatoms. The term"non-interfering" as used herein refers to substituents or linkinggroups that do not unduly interfere with either the photocuring reactionof the photocurable ionomer or its dark reaction with the reactivepowder.

Preferred X groups are acidic groups, with carboxyl groups beingparticularly preferred.

Suitable Y groups include, but are not limited to, polymerizableethylenically unsaturated groups and polymerizable epoxy groups.Ethylenically unsaturated groups are preferred, especially those thatcan be polymerized by means of a free radical mechanism, examples ofwhich are substituted and unsubstituted acrylates, methacrylates,alkenes and acrylamides. In aqueous systems, polymerizable groups thatare polymerized by a cationic mechanism, e.g., polymerizableethylenically unsaturated groups such as vinyl ether groups andpolymerizable epoxy groups, are less preferred since a free radicalmechanism is typically easier to employ in such systems than a cationicmechanism.

X and Y groups can be linked to the backbone B directly or by means ofany non-interfering organic linking group, such as substituted orunsubstituted alkyl, alkoxyalkyl, aryl, aryloxyalkyl, alkoxyaryl,aralkyl, or alkaryl groups.

Photocurable ionomers of Formula I can be prepared according to avariety of synthetic routes, including, but not limited to, (1) reactingn X groups of a polymer of the formula B(X)_(m+n) with a suitablecompound in order to form n pendent Y groups, (2) reacting a polymer ofthe formula B(X)_(m) at positions other than the X groups with asuitable compound in order to form n pendent Y groups, (3) reacting apolymer of the formula B(Y)_(m+n) or B(Y)_(n), either through Y groupsor at other positions, with a suitable compound in order to form mpendent X groups. and (4) copolymerizing appropriate monomers, e.g., amonomer containing one or more pendent X groups and a monomer containingone or more pendent Y groups.

The first synthetic route referred to above is preferred, i.e., thereaction of n X groups of a polymer of the formula B(X)_(m+n) to form npendent Y groups. Such groups can be reacted by the use of a "couplingcompound", i.e., a compound containing both a Y group and a reactivegroup capable of reacting with the polymer through an X group in orderto form a covalent bond between the coupling compound and the X group,thereby linking the Y group to the backbone B in a pendent fashion.Suitable coupling compounds are organic compounds, optionally containingnon-interfering substituents and/or non-interfering linking groupsbetween the Y group and the reactive group.

Particularly preferred photocurable ionomers of Formula I are those inwhich each X is a carboxyl group and each Y is an ethylenicallyunsaturated group that can be polymerized by a free radical mechanism.Such ionomers are conveniently prepared by reacting a polyalkenoic acid(e.g., a polymer of formula B(X)_(m+n) wherein each X is a carboxylgroup) with a coupling compound containing both an ethylenicallyunsaturated group and a group capable of reacting with a carboxylic acidgroup. The molecular weight of the resultant photocurable ionomers ispreferably between about 250 and about 500,000, and more preferablybetween about 5,000 and about 100,000. These ionomers are generallywater-soluble, but to a lesser extent than the polyalkenoic acids fromwhich they are derived. Hence, the use of cosolvents, as described morefully below, is preferred in order to enhance the solubility of theionomers and achieve more concentrated solutions thereof.

Suitable polyalkenoic acids for use in preparing ionomers of thisinvention include those homopolymers and copolymers of unsaturatedmono-, di-, or tricarboxylic acids commonly used to prepare glassionomer cements. Representative polyalkenoic acids are described, forexample, in U.S. Pat. Nos. 3,655,605, 4,016,124, 4,089,830, 4,143,018,4,342,677, 4,360,605 and 4,376,835.

Preferred polyalkenoic acids are those prepared by thehomopolymerization and copolymerization of unsaturated aliphaticcarboxylic acids, for example acrylic acid, 2-chloroacrylic acid,3-chloroacrylic acid, 2-bromoacrylic acid, 3-bromoacrylic acid,methacrylic acid, itaconic acid, maleic acid, glutaconic acid, aconiticacid, citraconic acid, mesaconic acid, fumaric acid and tiglic acid.Suitable monomers that can be copolymerized with the unsaturatedaliphatic carboxylic acids include unsaturated aliphatic compounds suchas acrylamide, acrylonitrile, vinyl chloride, allyl chloride, vinylacetate, and 2-hydroxyethyl methacrylate. Ter- and higher polymers maybe used if desired. Particularly preferred are the homopolymers andcopolymers of acrylic acid. The polyalkenoic acid should be surgicallyacceptable, that is, it should be substantially free from unpolymerizedmonomers and other undesirable components.

Particularly preferred polyalkenoic acids also include homopolymers ofpolyacrylic acid, and copolymers of acrylic and itaconic acids, acrylicand maleic acids, methyl vinyl ether and maleic anhydride or maleicacid, ethylene and maleic anhydride or maleic acid, and styrene andmaleic anhydride or maleic acid.

Polymers of formula B(X)_(m+n) can be prepared by copolymerizing anappropriate mixture of monomers and/or comonomers. Preferably, suchpolymers are prepared by free radical polymerization, e.g., in solution,in an emulsion, or interfacially. Such polymers can be reacted withcoupling compounds in the presence of appropriate catalysts, asdescribed more fully in the examples below.

Coupling compounds suitable for use for preparing the preferred ionomersof the present invention include compounds that contain at least onegroup capable of reacting with X in order to form a covalent bond, aswell as at least one polymerizable ethylenically unsaturated group. WhenX is carboxyl, a number of groups are capable of reacting with X,including both electrophilic and nucleophilic groups. Examples of suchgroups include the following moieties, and groups containing thesemoieties: --OH, --NH₂, --NCO, --COCl, and ##STR1##

Examples of suitable coupling compounds include, but are not limited to,acryloyl chloride, methacryloyl chloride, vinyl azalactone, allylisocyanate, 2-hydroxyethylmethacrylate, 2-aminoethylmethacrylate, and2-isocyanatoethyl methacrylate. Other examples of suitable couplingcompounds include those described in U.S. Pat. No. 4,035,321, thedisclosure of which is hereby incorporated by reference. Examples ofpreferred coupling compounds include, but are not limited to, thefollowing methacrylate compounds and their corresponding acrylates##STR2##

as well as the following allyl compound ##STR3##

Particularly preferred coupling compounds are the following methacrylatecompounds and their corresponding acrylates, wherein R is as definedabove. ##STR4##

wherein q is 1 to 18. ##STR5##

wherein q is as defined above, ##STR6##

Preferred photocurable ionomers of Formula I are prepared by reacting apolymer of formula B(X)_(m+n) wherein X is COOH with a coupling compoundcontaining a reactive group of the formula NCO. The resultant ionomers,e.g., those of Formula I above wherein the covalent bond between the Xgroup and the reactive group of the coupling compound is an amidelinkage, are believed novel and provide an optimal combination of suchproperties as adhesion to dentin, mechanical strength, working time,fluoride release and the like.

The preferred photocurable ionomers of the present invention can beformulated in water, either alone or with the use of adjuvants such ascosolvents described in greater detail below. The preferredconcentration of ionomer in aqueous solution is between about 10 andabout 70 percent by weight, based on the weight of the final aqueoussolution, and more preferably is between about 20 and about 50 percentby weight. For optimal use in preparing a cement of the presentinvention, the preferred viscosity of the ionomer solution is betweenabout 60 and about 900 centistokes, and most preferably between about150 and about 500 centistokes. Ionomer solutions having higherviscosities will generally be more difficult to mix, and solutions oflower molecular weight ionomer will generally provide cements havinglower strength.

In order to prepare a photocurable ionomer cement from the cement systemof this invention, a photocurable ionomer is mixed with a reactivepowder in the presence of water. Optionally, and preferably, the cementsystem also includes modifying agent and polymerization initiator,thereby providing the ability to achieve a longer working time and ashorter setting time, respectively, when preparing the resultant cement.

Reactive powders suitable for use in the cement systems of thisinvention include those that are commonly used with ionomers to formionomer cements. Examples of suitable reactive powders are described inthe Prosser et al. article cited above, the disclosure of which ishereby incorporated by reference, as well as metal oxides such as zincoxide and magnesium oxide, and ion-leachable glasses, e.g., as describedin U.S. Pat. Nos. 3,655,605, 3,814,717, 4,143,018, 4,209,434, 4,360,605and 4,376,835.

Particularly preferred reactive powders for use in the cement systems ofthis invention are those that contain leachable fluoride, since thesustained release of fluoride ions as a byproduct of the settingreactions provides cariostatic benefits. Examples of preferred powdersinclude fluoroaluminosilicate and fluoroaluminoborate ion-leachableglasses.

The ionomer cement systems of the invention can frequently bepolymerized without the use of one or more polymerization initiators,e.g., by the use of thermal energy or by exposure to a high energypulsed xenon source. Optionally, and preferably, the ionomer cementsystem contains one or more suitable polymerization initiators that actas a source of free radicals when activated. Such initiators can be usedalone or in combination with one or more accelerators and/orsensitizers.

Polymerization initiators suitable for use in the present inventioninclude electromagnetic radiation-induced polymerization initiators,such as ultraviolet- or visible-light-induced polymerization initiators,that exhibit a desired combination of such properties as stability andefficiency of free radical production and polymerization initiation.

Examples of suitable ultraviolet-induced polymerization initiatorsinclude, but are not limited to, ketones such as benzil and benzoin, andacyloins and acyloin ethers, commercially available, for example, fromAldrich Chemical Co. Preferred ultraviolet-induced polymerizationinitiators include 2,2-dimethoxy-2-phenylacetophenone ("Irgacure 651 ")and benzoin methyl ether (2-methoxy-2-phenylacetophenone), bothcommercially available from Ciba-Geigy Corp.

Examples of suitable visible-light-induced initiators include, but arenot limited to, diaryliodonium salts and triarylsulfonium salts, as wellas chromophore substituted halomethyl-s-triazines, such as thosedescribed in U.S. Pat. No. 3,954,475, and halomethyl oxadiazoles such asthose described in U.S. Pat. No. 4,212,970. Such initiators can be usedalone or in combination with suitable accelerators, e.g., amines,peroxides, and phosphorus compounds, and/or with suitablephotosensitizers, e.g., ketone or alpha-diketone compounds.

For photocurable ionomers that are polymerized by a cationic mechanism,suitable initiators include salts that are capable of generating cationssuch as the diaryliodonium, triarylsulfonium and aryldiazonium salts.

Preferred visible light-induced polymerization initiator systems includesuitable combinations of a diketone, e.g., camphorquinone, and adiaryliodonium salt, e.g., diphenyliodonium chloride, bromide, iodide orhexafluorophosphate, with or without additional hydrogen donors, oraccelerators, such as sodium benzene sulfinate, amines or aminealcohols.

Polymerization initiator, when employed, is preferably present in theionomer cement system in an amount sufficient to achieve the desiredextent of polymerization. Such amount is dependent in part on theextinction coefficient of the initiator and the thickness of the layerto be exposed to radiant energy. Typically, an ultraviolet-inducedpolymerization initiator will be present at about 0.01% to about 5%,based on the weight of the ionomer(s) present, and the components of avisible light-induced polymerization initiator system will generally bepresent at a combined weight of about 0.01 to 5%, and preferably fromabout 0.1 to 5%, based on the weight of the ionomer(s) present.

The components of the photocurable ionomer cement system can becombined, e.g., blended or mixed, in a variety of manners and amounts inorder to form the photocurable ionomer cement of this invention.Suitable combining techniques include those commonly employed to mixionomer cement systems.

In one suitable technique, a concentrated aqueous solution ofphotocurable ionomer is mixed with reactive powder at the time of use.The resultant combination of ionomer, powder and water allows thesetting reaction to begin.

In an alternative technique, the photocurable ionomer and powder areprovided as a powdered blend under substantially anhydrous conditions,i.e., conditions in which there is not sufficient water to allow thesetting reaction to proceed. Such systems can then be combined withwater at the time of use in order to begin the setting reaction.

The ratio of powder (i.e., reactive powder or powdered blend of ionomerand reactive powder) to liquid in such techniques is an important factorin determining the workability of the mixed ionomer cement systems.Ratios higher than about twenty to one (powder to liquid, by weight)tend to exhibit poor workability, while ratios below about one to onetend to exhibit poor mechanical properties, e.g., strength, and henceare not preferred. Preferred ratios are on the order of about one to oneto about five to one.

Optional other ingredients, such as polymerization initiators, modifyingagents and cosolvents can be added at any time and in any manner thatdoes not prematurely begin the setting reaction or the photocuringreaction. Modifying agents can be used in the ionomer cement systems ofthe present invention in order to provide prolonged working times.Applicant has discovered a new group of compounds useful as modifyingagents in the systems of the present invention.

Modifying agents useful in the cement system of the present inventionare selected from the group consisting of alkanolamines, e.g.,ethanolamine and triethanolamine, and mono-, di- and tri-sodiumhydrogenphosphates.

Modifying agents can either be incorporated into an aqueous solution ofthe ionomer, or can be milled with the powder to be used in the ionomercement system. The modifying agents are preferably used at aconcentration between about 0.1 to about 10 percent by weight, based onthe weight of the reactive powder, and preferably between about 0.5 toabout 5 percent.

Cosolvents useful in the present invention include, but are not limitedto, low molecular weight organic solvents. The word "cosolvent", as usedherein refers to a material that aids in the dissolution of aphotocurable ionomer in water, in order to form a homogeneous aqueoussolution of cosolvent and ionomer. Suitable cosolvents includenon-copolymerizable organic solvents and copolymerizable low molecularweight hydrophilic alkenyl solvents. The word "copolymerizable" as usedherein refers to the ability of the cosolvent to cure compatibly withthe ionomers used in the invention. Copolymerizable cosolvents can beadded to the ionomer cement systems of this invention for a variety ofreasons, for instance, to provide a homogeneous solution of aphotocurable ionomer having inherently low aqueous solubility, toshorten the exposure of radiant energy needed to cure the system, or tovary the physical properties, e.g., the flexibility, of the resultantcured ionomer cement. Examples of suitable cosolvents includenon-copolymerizable cosolvents such as ethanol, propanol, and glycerol,and copolymerizable cosolvents such as 2-hydroxylethylmethacrylate or2-hydroxypropyl-methacrylate.

Sufficient amounts of each component in the cement systems of thepresent invention should be employed to obtain the desired working time.Preferably such systems will provide a working time of at least aboutone minute and most preferably greater than two minutes, during whichtime the systems can be cured by exposure to an appropriate source ofradiant energy. For the sake of brevity this discussion will focus ondental applications, and particularly, the curing of such systems insitu, e.g., in the mouth of a patient.

The curing of the ionomer cement system is accomplished by exposure toany source of radiant energy capable of causing the desired extent ofpolymerization of the photocurable ionomer. Suitable radiant energysources afford a desired combination of such properties as safety,controllability, suitable intensity, and suitable distribution ofincident energy. See generally, "Radiation Curing", Kirk-othmerEncyclopedia of Chemical Technology 3d Ed., Vol. 19, pp. 607-624 (1982).Preferred radiant energy sources are ultraviolet or visible lightsources whose emission spectra correspond closely with the absorptionrange of the polymerization initiator in the ionomer cement system. Forinstance, sources emitting ultraviolet light at wavelengths betweenabout 335 and 385 nm, and sources emitting visible light in the blueregion at wavelengths between about 420 and 480 nm are preferred for usewith the preferred ultraviolet- and visible-light-induced polymerizationinitiators, respectively. For polymerizing cement systems in the mouth,visible light radiation such as that provided by standard dental curinglights is particularly preferred.

Upon exposure of an ionomer cement system of the present invention to anappropriate source of radiant energy, the system rapidly begins to cure,e.g., within about 45 seconds, and preferably within about 30 seconds.The restoration generally exhibits the greatest degree of cure at itssurface, where the radiant energy is most intense. The surface of therestoration therefore can be cured sufficiently to allow subsequentprocedures to be performed on the restoration, while the interior of therestoration is allowed to harden fully by means of the ongoing settingreaction. Thus, if the curing step is omitted, the usual settingreaction will occur, ultimately resulting in the hardening of thematerial, even in the dark. This phenomenon offers a unique advantage inthat a relatively deep restoration can be prepared by rapidly curing theouter surface of the restoration instantly by exposure to radiantenergy, allowing the inner portions of the restoration to cure moreslowly by the usual setting reaction. As a result, the dentist cancontinue to carry out further restorative procedures, e.g., layeringfurther ionomer cement on the hardened surface, while the inner portionscontinue to harden. This can result in a substantial saving of time forthe practitioner and patient.

The ionomer cements of this invention can be used in a variety ofapplications in the dental or medical fields in which a bulk curablematerial of low shrinkage is desired that will adhere well to thesurrounding tooth or bone structure. For instance, these cements can beused as dental restoratives for lining or basing Class I, II, III and Vrestorations, for cementation, as sealants, and as bulk fillingmaterials.

The present invention will be further understood in view of thefollowing examples which are merely illustrative and not meant to limitthe scope of the invention. Unless otherwise indicated, all parts andpercentages are by weight.

EXAMPLE 1

Synthesis of Low Molecular Weight Polyacrylic Acid

A glass reactor fitted with two addition funnels, a thermometer, amechanical stirrer, a reflux condenser and a nitrogen inlet tube wascharged with 354.4 parts dry tetrahydrofuran ("THF") (water content<0.02%). A solution of 144 parts acrylic acid monomer in 82.4 parts THFwas charged into one of the addition funnels. A solution of 1.64 partsazobisisobutyronitrile ("AIBN") initiator in 102 parts THF was chargedinto the second funnel. The nitrogen purge was started and the reactorheated. When a temperature of approximately 60° C. was attained in thereactor vessel, the monomer solution was added at a rate of about 9parts every 5 minutes and initiator solution was added at a rate ofabout 4.5 parts every 5 minutes. After the additions were complete, thereaction was allowed to proceed at about 60° C. for an additional 2hours, resulting in a homogeneous, slightly hazy solution. Gelpermeation chromatography ("GPC") showed the weight average molecularweight (M_(w)) of the resultant polymer to be 9,700 with apolydispersity of 2.7.

EXAMPLE 2

Synthesis of High Molecular Weight Polyacrylic Acid

Nitrogen gas was bubbled into a solution of 15 parts acrylic acid, 82.5parts p-dioxane and 0.15 parts of AIBN for a period of 15 minutes. Thereaction vessel was then stoppered and heated at about 60° C. forapproximately 18 hours, at which time infrared spectral analysis showedthe absence of C═C bands at 1635 cm⁻¹. Gel permeation chromatography ofthe homogeneous, clear, viscous product showed the M_(w) to be 115,452with a polydispersity of 4.48.

EXAMPLE 3

Synthesis of Copoly 4:1(Acrylic:Itaconic) Acid

The reactor of EXAMPLE 1 was charged with 132.9 parts THF. One of theaddition funnels was charged with a monomer solution containing 58.6parts acrylic acid, 26.0 parts itaconic acid and 150.6 parts THF. Theother addition funnel was charged with an initiator solution containing0.82 parts AIBN in 115 parts THF. The reactor vessel was flushed withnitrogen and heated to about 60° C. The monomer solution was added at arate of about 9 parts every 15 minutes and the initiator solution wasadded at a rate of about 4.5 parts every 15 minutes. The temperature ofthe reactor vessel was kept at about 62-64° C. After the addition ofmonomer and initiator solutions was complete, the reaction mixture wasallowed to stir at approximately about 64° C. for approximately 17hours, at which time infrared spectral analysis showed that thepolymerization reaction was complete.

EXAMPLE 4

Synthesis of Copoly 7:3(Acrylic:Itaconic) Acid

The reactor of EXAMPLE 1 was charged with 134 parts THF and flushed withnitrogen. A monomer solution containing 39 parts itaconic acid, 50.4parts acrylic acid and 226 parts THF was added at a rate of about 12parts every 5 minutes. An initiator solution consisting of 0.82 partsAIBN in 51 parts THF was added at a rate of about 2.2 parts every 5minutes. After addition was complete, the reaction mixture was heated atabout 60° C. for approximately 2 hours. Gel permeation chromatographyshowed the M_(w) to be 18,310 with a polydispersity of 3.0.

EXAMPLE 5

Synthesis of Copoly 4:1(Acrylic:Maleic) Acid

The reactor of EXAMPLE 1 was charged with 268 parts THF and flushed withnitrogen. A monomer solution containing 23.2 parts maleic acid, 57.6parts acrylic acid and 88.6 parts THF, was added at a rate of about 6.6parts every 5 minutes. An initiator solution containing 0.82 parts AIBNin 51.4 parts THF was added at a rate of about 2.2 parts every 5minutes. The reaction mixture was then allowed to stir for an additional2 hours at about 60° C. Gel permeation chromatography showed the M_(w)to be 10,800 with a polydispersity of 2.5.

EXAMPLES 6-8

Reaction of Polyacrylic Acid of EXAMPLE 1 with 2-IsocyanatoethylMethacrylate

Into a three-necked glass reaction vessel fitted with mechanicalstirrer, dry air inlet tube, addition funnel and thermometer wastransferred a portion of the THF solution of EXAMPLE 1 which contained24.7 parts polyacrylic acid. To this solution were added sequentially0.08 parts BHT, 0.08 parts triphenylstibine ("TPS"), 0.135 partsdibutyltin dilaurate ("DBTL"), and an additional 26.6 parts THF. Thestirrer was started and the reaction mixture was heated to about 32-35°C. The amount of 2-isocyanatoethyl methacrylate ("IEM") for each Examplewas varied as shown in TABLE I. The IEM was added dropwise over a periodof approximately 45-50 minutes, so that the reaction temperature did notexceed about 40° C. After the IEM addition was complete, the reactionwas stirred at this temperature until the evolution of carbon dioxideceased. At this point, the heating source was removed and the reactionwas allowed to stir at about 20° C. for an additional hour. Infraredspectral analysis showed the absence of the NCO band at 2350 cm⁻¹ andthe presence of the amide band at 1530 cm⁻¹. The homogeneous solutionwas then transferred to a rotary evaporator and concentrated to a syrupyconsistency. The concentrate was added in a thin stream to approximately500 parts of diethyl ether with agitation, whereupon the polymerprecipitated as a fine white solid. The precipitate was filtered, washedwith 100 parts of diethyl ether and dried in vacuo. Set out below inTABLE I are the parts of IEM, the yield of polymer, and the viscosity ofa 45% solution of the polymer in a mixture of 2-hydroxyethylmethacrylate ("HEMA")/water (2:3 by weight).

TABLE I

    ______________________________________                         Polymer    Example  Parts of IEM                         Yield (%)                                  Viscosity (cstokes)    ______________________________________    6        6.38        100      233    7        7.97        99.9     286    8        9.56        96.5     262    ______________________________________

EXAMPLE 9

Reaction of Polyacrylic Acid with Allyl Isocyanate

To a solution containing 2.3 parts of the polyacrylic acid prepared asdescribed in EXAMPLE 2 was added 0.005 parts of BHT and 0.01 parts ofDBTL. The mixture was stirred to obtain a clear solution. A solution of0.6 parts of allyl isocyanate in 2 parts of p-dioxane was addeddropwise. The reaction mixture was allowed to stir at about 60° C. untilthe evolution of CO₂ ceased. The mixture was then cooled to about 20° C.and allowed to stir for an additional 18 hours. The polymer wasprecipitated, filtered, washed with hexanes and dried.

EXAMPLE 10

Reaction of Polyacrylic Acid with IEM

2 Parts of DBTL were added, with stirring, to 15 parts of thepolyacrylic acid solution of EXAMPLE 2. IEM (7.5 parts) containing 0.05parts BHT was added dropwise to the mixture. The reaction mixture wasstirred at approximately 20° C. for about 1/2 hour, followed by heatingat about 60° C. for approximately 1 hour. Copious evolution of carbondioxide was observed initially, but ceased as the reaction approachedcompletion. A white material precipitated out initially, but withcontinued stirring at about 20° C., it gradually dissolved providing aclear solution. The solution was added in a slow stream to diethylether; the solid which precipitated was filtered, washed with diethylether and dried in vacuo. The dry polymer was dissolved in a 2:3 mixtureof HEMA and water.

EXAMPLE 11

Reaction of Copoly(4:1)(Acrylic:Itaconic Acid) with IEM

The polymerized reaction mixture of EXAMPLE 3 was allowed to cool toabout 35° C. To the stirred mixture was added 0.15 parts BHT, 0.15 partsTPS and 1.03 parts DBTL. A stream of air was introduced into thereaction mixture, and the reaction temperature was increased to about40° C. A solution of 35.34 parts IEM dissolved in 22 parts THF was addeddropwise over a period of about 1 1/2 hours. The reaction mixture wasthen allowed to stir at about 40° C. for an additional hour, followed bystirring at about 20° C. for approximately 18 hours. The homogeneoussolution was concentrated to a syrupy consistency. It was thenprecipitated into five times its volume of ethyl acetate. Theprecipitate was filtered, washed with ethyl acetate and dried in vacuo.The polymer yield was 98%. The dry polymer (45 parts) was dissolved in amixture containing 33 parts of water and 22 parts of HEMA to yield ahomogeneous solution having a viscosity of 276 cstokes.

EXAMPLE 12

Preparation of Light Cure Ionomer Solution

To dried ionomers (prepared from 10 parts of the polyacrylic aciddescribed in EXAMPLE 1, and reacted with 2.08 parts IEM as described inEXAMPLES 6-8) were added HEMA and distilled water in the amounts shownin TABLE II. The viscosity of each of the resultant homogeneoussolutions was measured.

                  TABLE II    ______________________________________            Weight % of      Viscosity    Example   Ionomer HEMA       Water (cstokes)    ______________________________________    12a       25      28         47    26    12b       25      41         34    35    12c       35      31         34    125    12d       45      21         34    181    12e       45      30         25    185    ______________________________________

Each of the above ionomer solutions was combined with a polymerizationinitiator system as follows:

    ______________________________________    Ionomer solution  4.346 parts    Camphorquinone    0.021 parts    Diphenyliodonium chloride                      0.135 parts    ______________________________________

EXAMPLE 13

Measurement of Adhesion to Dentin

Adhesion of light-curable ionomer cement systems to bovine dentin wasmeasured using the following procedure:

1. Apply mixed ionomer cement system to freshly polished (600 grit)bovine dentin.

2. Cure for 20 seconds with dental curing light ("Visilux 2", 3M).

3. Apply dental adhesive ("Scotchbond™ Dual Cure", 3M).

4. Cure for 20 seconds.

5. Apply light-curable restorative ("P-30", 3M) by molding in the shapeof a button.

6. Cure for 20 seconds.

7. Store in water at 37° C. for 24 hours.

8. Shear off button in a tensile tester ("Instron") at a crosshead speedof 2 mm/min.

A fluoroaluminosilicate glass frit was prepared by fusing together andthen cooling the following ingredients.

    ______________________________________            Ingredient                   Parts    ______________________________________            SiO.sub.2                   26.84            Al.sub.2 O.sub.3                   0.80            P.sub.2 O.sub.5                   0.94            NH.sub.4 F                   3.32            AlF.sub.3                   20.66            Na.sub.2 AlF.sub.6                   10.65            ZnO    20.66            MgO    2.12            SrO    12.55    ______________________________________

The resulting frit was comminuted to give a fine powder which was thenscreened through a 44 micron mesh screen. Surface area was determined tobe 1.1 m² /g using a "Monasorb" dynamic flow, single point BET surfacearea analyzer (Quantachrome Co., Syosset, N.Y.).

Ionomer solutions prepared and combined with a polymerization initiatorsystem as described above in EXAMPLE 12 were mixed with the glass powderat a powder:liquid ratio of 1.4:1 and hand spatulated for approximately15 seconds at about 20° C. to give a smooth creamy mix. Adhesion resultsare shown in TABLE III below.

                  TABLE III    ______________________________________    Ionomer     Adhesion (kg/cm.sub.2)    ______________________________________    12a         68.0    12b         115    12c         125    12d         115    12e         91    ______________________________________

In contrast, the adhesion values of commercially available ionomercement systems, namely "GC Lining Cement", GC Dental Corp., Tokyo,Japan, and "Ketac Bond", Espe Fabrik Pharm. Gmbh, West Germany, weredetermined by the same method to be 40 kg/cm² and 45 kg/cm²,respectively. The results in TABLE III indicate that the adhesion valuesof cements prepared from the photocurable ionomer cement systems of thepresent invention can substantially exceed the adhesion values of thecomparable commercially available ionomer cements.

EXAMPLE 14

Effect of Additives in the Reactive Powder

The glass frit prepared as described in EXAMPLE 13 was combined withvarying amounts of disodium hydrogen phosphate and milled to form apowder (using 12 mm×12 mm alumina rod media) in a ceramic jar rotated at60 rpm for about 3 hours. Surface area was determined as described inEXAMPLE 13. The powder was then slurried in a solution containing 1 partdiphenyliodonium chloride (polymerization initiator) and 99 partsmethanol. The solvent was then evaporated and the dry,initiator-containing powder was screened through a 74 micron meshscreen. Ionomer solutions (prepared as described in EXAMPLE 12d andcombined with 0.5% camphorquinone) were mixed with initiator-containingpowder at a powder:liquid ratio of 1.4:1 by weight and hand spatulatedat approximately 20° C. so as to blend the mixture thoroughly in about15 seconds using about 30 strokes.

Working time was evaluated by rapidly molding the mixed cement with aspatula into a bead approximately 2.5 cm long and 0.6 cm wide. Fortyseconds from the start of the mix a ball applicator was drawnperpendicularly through the bead allowing cement to be pulled across themixing pad. This procedure was repeated every 10-20 seconds until thecement became excessively stringy or unworkable. Adhesion was determinedas described in EXAMPLE 13.

For diametral tensile and compressive strength measurements the mixedcement samples were injected into a glass tube having a 4 mm innerdiameter. The filled tube was placed on a vibrator for 30 seconds toeliminate trapped air bubbles, then subjected to 2.88 kg/cm² (40 psi)pressure followed by curing while under pressure, by exposure to aVisilux 2 dental curing light. The cured samples were allowed to standfor 1 hour at about 37° C., 90%+ relative humidity. They were then cuton a diamond saw to form cylindrical plugs 2 mm long for measurement ofdiametral tensile strength, and 8 mm long for measurement of compressivestrength. The plugs were stored in distilled water at approximately 37°C. for about 24 hours and their diametral tensile and compressivestrengths were determined according to ISO specification 7489.

The composition of three powders and the concentration of disodiumhydrogen phosphate, together with the properties observed for eachsample are provided in TABLE IV.

                  TABLE IV    ______________________________________                     Ex. 14a                           Ex. 14b   Ex. 14c    ______________________________________    % Na.sub.2 HPO.sub.4                        0      1         2    Surface area (m.sup.2 /g)                       1.39    1.24      1.17    Working time (seconds)                       35      135       260    Compressive strength (MPa)                       --      93.5      68.9    Diametral tensile strength (MPa)                       --      18.1      11.6    Adhesion to dentin (kg/cm.sup.2)                       89      114       118    ______________________________________

The results in TABLE IV indicate that working time can be greatlyaffected, e.g., prolonged, by the adjustment of the concentration ofmodifying agent. In this case, the modifying agent is a compound thatreacts with the powder in a manner that competes with, and therebydelays the acid-base setting reaction involving the ionomer and powder.The diametral tensile and compressive strengths are somewhat lowered byvirture of such competing reactions, although still remain well withinacceptable limits. The adhesion values were not significantly affected.

EXAMPLE 15

Comparison of Various Additives as Modifying Agents

A solution of ionomer prepared as described in EXAMPLE 11 and combinedwith a polymerization initiator system as described in EXAMPLE 12 wasmixed with various additives as shown in TABLES V and VI in an attemptto modify working times of ionomer cement systems of the presentinvention. The resulting solutions were mixed with a powder obtained bycomminuting the glass composition prepared as described in EXAMPLE 13.Working times were determined as described above in EXAMPLE 14. Adhesionwas determined as described in EXAMPLE 13.

                  TABLE V    ______________________________________    Effect of Additives on Working Time    Powder surface area 1.04 m.sup.2 /g    Powder:Liquid ("P/L") ratio 1.4:1    Additive   Concentration (w/w %)                              Working Time (sec)    ______________________________________    None       --             100-120    Tartaric acid               1              70    Tartaric acid               3              35    Citric acid               1              60-90    Citric acid               3              60-90    Citric acid               5              40    EDTA       1              60-75    Disodium EDTA               1              80    H.sub.3 PO.sub.4               3              100-120    H.sub.3 PO.sub.4               6              60    H.sub.3 PO.sub.4               12             10    ______________________________________

                  TABLE VI    ______________________________________    Effect of Additives on Working Time and Adhesion    Powder surface area 1.4 m.sup.2 /g    P/L ratio 1.4:1    Concentration         Working Time                                     Adhesion    Additive    w/w %     (sec)      kg/cm.sup.2    ______________________________________    None        --        40-60      94    Ethanolamine                3         125        --    Triethanolamine                5         200        --    NaH.sub.2 PO.sub.4 •H.sub.2 O                3         390        78    NaH.sub.2 PO.sub.4 •H.sub.2 O                5         325        86    Na.sub.2 HPO.sub.4                1         120        107    Na.sub.2 HPO.sub.4                3         330        100    Na.sub.2 HPO.sub.4                5         450        65    Na.sub.3 PO.sub.4 •12 H.sub.2 O                  7.8     400        --    ______________________________________

Comparison of the data in TABLES V and VI show that acidic additivestend to decrease working time whereas basic additives tend to increaseworking time.

EXAMPLE 16

Determination of Properties of Set Cements

Polymerization initiator-containing glass powder prepared as describedin EXAMPLE 14 was mixed with the ionomer solutions described in EXAMPLES6-8 (formulated with 0.5k camphorquinone, based on the weight of theliquid), at a powder to liquid weight ratio of 1.4 to 1. The workingtime and the properties of the set cement after curing by 20 secondexposure to a Visilux 2 dental curing light are set forth below in TABLEVII. A commercially available ionomer cement, "GC" brand lining cement,available from GC Dental Corp., was prepared according to its directionsas a comparative sample.

                  TABLE VII    ______________________________________                  Liquids                  Ex. 6                       Ex. 7    Ex. 8  GC    ______________________________________    Working time (min:sec)                    3:15   3:45     4:10 1:30    Adhesion to dentin (kg/cm.sup.2)                    104.5  99.5     101.2                                         40    Compressive strength (MPa)                    63.7   67.5     53.2 59.0    Diametral tensile strength (MPa)                    11.4   12.4     11.2 5.6    ______________________________________

The results in TABLE VII indicate that cements of the present inventionexhibited equivalent compressive strength and superior adhesion anddiametral tensile strength compared to the conventional cement.

EXAMPLE 17

Preparation of Photocurable Ionomers having Ester Linkages

A 15% solution of poly(vinylazalactone) was prepared according to theprocedure described in Heilman et al., "Chemistry of Alkenyl AzalactonesI. Radiation Sensitive Materials derived from Azlactone ContainingCopolymers", J. Polym. Sci., Polym. Chem. Ed. 22, 1179-1186 (1984). Thepolymer was isolated by precipitating 50 parts of the above solution in300 parts of mixed hexanes and redissolving the isolated polymer in 24parts of dry acetone. To this solution were added 1.31 parts of HEMA,0.02 parts 4-methoxyphenol ("MEHQ") and 0.15 parts of trifluoroaceticacid. The reaction mixture was heated at approximately 60° C. Afterabout 21 hours the azalactone peak at 1820 cm⁻¹ decreased due to amideformation, showing that addition of HEMA had occurred. To 18.9 parts ofthe resulting reaction product were added 0.43 parts of water and 3parts of trifluoroacetic acid and the resultant mixture was allowed tostir at approximately 20° C. After about 3 days the reaction mixture wasconsiderably more viscous than initially observed. Infrared spectralanalysis showed that the azalactone peak had disappeared completely andwas replaced by a CO₂ H peak in the 3000-2500 cm⁻¹ region. The polymerwas then precipitated in hexanes, filtered and dried in vacuo. A portion(1.09 parts by weight) of the dry polymer was dissolved in 0.80 parts ofHEMA and 0.67 parts of polyacrylic acid solution ("Good-rite K-732", B.F. Goodrich, Cleveland, Ohio). To the resulting solution were added0.091 parts diphenyliodonium chloride and 0.0156 parts camphorquinone.One part of solution was mixed with 1.2 parts of glass powder preparedas described in EXAMPLE 13 and the mixture was irradiated with Visilux 2dental curing light for 30 seconds. A hard mass was obtained.

EXAMPLE 18

Preparation of Methylvinylether-containing Photocurable Ionomers

To a mixture of 1.77 parts of methylvinylether maleic anhydridecopolymer ("Gantrez AN119", GAF Corp., New York, N.Y.) in 51 parts ofdry tetrahydrofuran was added (1) a solution consisting of 0.34 parts ofHEMA, 0.009 parts of MEHQ and 1.6 parts of THF, followed by (2) asolution of 0.015 parts of 1,4 diazabicyclo(2.2;2)octane ("DABCO",Aldrich Chemical Co.) catalyst dissolved in 1.6 parts of THF. Themixture was heated under reflux for about 21 1/2 hours, cooled toapproximately 20° C. and then precipitated in hexanes. The pale pinkprecipitate was filtered, washed with hexanes and dried in vacuo.Infrared and nuclear magnetic resonance spectral analyses indicated thatethylenically unsaturated groups had been linked to the polycarboxylicacid.

EXAMPLE 19

Fluoride Release

In vitro fluoride release was measured for a sample of the cured cementmix of EXAMPLE 13 using the ionomer of EXAMPLE 12c in phosphate bufferof pH 6.4 using a fluoride ion selective electrode according to methodof Swartz et al., J. Dent. Res., 63, 158-160 (1984). TABLE VIII belowshows the cumulative amount of fluoride leached per gram of the lightcured glass ionomer sample, compared with a conventional commerciallyavailable glass ionomer cement Ketac Bond (Espe).

                  TABLE VIII    ______________________________________                Fluoride release (μg/g)    Days          Sample  Ketac Bond    ______________________________________    1             186     83    2             176.5   161    3             252     169    4             370     198    7             550     298    11            715     379    17            900     463    25            1110    657    31            1235    740    46            1665    871    74            1791    959    102           1950    1036    137           2450    1140    203           2984    1084    ______________________________________

The light-curable glass ionomer cement system was evaluated in vitro forcariostatic activity using the method developed by R. L. Erickson etal., J. Dent. Res., 66, Abstract No. 1114 (1987). It was found tocontain a substantial caries inhibition zone thus indicating that thematerial should be cariostatic.

I claim:
 1. A photocurable ionomer of the formula

    B(X).sub.m (Y).sub.n

wherein B represents an organic backbone; each X independently is anionic group capable of undergoing a setting reaction in the presence ofwater and a reactive powder, each Y independently is a photocurablegroup selected from the group consisting of free radically photocurableethylenically unsatureated groups, cationically photocurable vinyl ethergroups and cationically photocurable epoxy groups, m is a number havingan average value of 2 or more, and n is a number having an average valueof 1 or more, wherein at least one of said Y groups is linked to saidbackbone by means of an amide linkage formed by reacting an X group ofthe formula --COOH of a polymer of the formula B(X)_(m+n) with acoupling compound containing both said Y group and a group of theformula --NCO capable of reacting with said X group.
 2. A photocurableionomer of the formulaB(X)_(m) (Y)_(n) wherein B represents an organicbackbone; each X independently is an ionic group capable of undergoing asetting reaction in the presence of water and a reactive powder, each Yindependently is a photocurable group selected from the group consistingof free radically photocurable ethylencially unsatureated groups,cationically photocurable vinyl ether groups and cationicallyphotocurable epoxy groups, m is a number having an average value of 2 ormore, and n is a number having an average value of 1 or more, whereinsubstantially all of said Y groups are linked to said backbone by meansof an amide linkage formed by reacting an X group of the formula --COOHof a polymer of the formula B(X)_(m+n) with a coupling compoundcontaining both said Y group and a group of the formula --NCO capable ofreacting with said X group, wherein said Y group linked by means of anamide linkage is the same as the Y group that is derived from thereaction of a polymer of the formula B(X)_(m+n) with a coupling compoundselected from the group consisting of allyl isocyanate,2-isocyanatoethyl methacrylate, ##STR7## wherein p is 1 to 20 and R is Hor lower alkyl (e.g., having 1 to 6 carbon atoms), ##STR8## wherein q is1 to 18, ##STR9## wherein q is as defined above, ##STR10## and theircorresponding acrylates.
 3. An ionomer according to claim 2 wherein theweight average molecular weight of said ionomer is between about 250 andabout 500,000.
 4. An ionomer according to claim 2 wherein said molecularweight is between about 5,000 and about 100,000.